SURVEY OF THE BRITISH ISLES FOR THE EPOCH JANUARY I, 1886. 
59 
To obviate, as far as possible, the use of decimals, the Forces are throughout this 
paper expressed in terms of metric units (metre, gram, second), and all numerical 
values of Forces can be reduced to C.G.S. units by dividing by 10. 
In the case of the Dips, of which no continuous record is kept, the following plan 
was adopted. We give, under the column S, our results corrected for diurnal variation 
and disturbance as hereafter described, each being the mean of the two needles. In 
column K are the values obtained at Kew at the nearest date to the time of our 
observations taken from the published record. The dates in question are July 28 and 
31, 1884, Sept. 22, 24, and October 25, 26, 1887. The mean of the last two is taken 
as applying to the whole period of the 1887 observations. 
Inclination. 
Date. 
Instrument. 
Observer. 
S 
(Survey 
Instrument). 
K 
(Kew 
Instrument). 
K-S = /3. 
u 
II 
1 
M 
S-K'. 
July 17, 1884 . 
74 
R 
6°7 .se'-o 
67 38-8 
2'8 
67 36'1 
- OT 
„ 18, „ . 
T 
67 35-8 
3-0 
, * 
- 0-3 
„ 19, „ 
R 
67 36-4 
2-4 
, , 
+ 0-3 
Sept. 30, 1887 . 
99 
67 34-9 
67 37-5 
2-6 
67 34-8 
+ 0-1 
Oct. 11, ,, 
83 
T 
67 34-8 
, ^ 
2-7 
. , 
0-0 
„ 13, „ . 
67 35-0 
2-5 
+ 0-2 
„ 18, ,, 
9 ? 
99 
67 34-2 
3-3 
— 0-6 
„ 19, „ . 
74 
R 
67 34-9 
2-6 
+ 0-1 
l3 = 2-7 
Mean . . 
+ 0-2 
The mirrors used to form an image of the Sun which can be viewed through the 
telescope were tested independently. We were, however, unfortunate in that on 
most of the days when we observed at Kew the Sun was invisible. The meiidian 
mark also, which is used in the Kew measurements, cannot be seen from the lawn 
immediately in front of the Magnetic House, and we were, therefore, obliged to use a 
rod which was fixed at some distance, as nearly as might be in the meridian line in 
which the instrument was also placed. On one occasion (October 11, 1887) the latter 
adjustment was inaccurate, and a correction was applied deduced from the angle 
subtended at the pillar in the magnetic house by the centre of the tripod and the rod, 
and from the distances of the tripod and pillar from the rod. At Parsonstown the 
instrument was placed in the meridian line used for the Observatory of the Earl of 
Posse, and a check on the declinometer was thus obtained. From what has been said 
it will liave been seen that the determinations of the geographical meridian at Kew 
were not made under the most favourable conditions, but the following Table is 
sufficient to show that no important error attached to the indications of the 
i nstruments. 
I 2 
